1. Field of the Invention
This invention relates to the field of high speed optical delay lines and fiber-optic multiplexers.
2. Description of the Prior Art
The precise control of the relative timing of multiple pulse trains is often a critical factor in the synchronization of different components in a system, such as, for example, in a time-multiplexing application. The use of variable time delay is a convenient means of providing this phase control. Conventional techniques, such as the utilization of a predetermined length of coaxial cable, are currently used to provide this phase control. However, for high-speed (multi-gigahertz) signals, conventional coaxial cable delay lines experience capacitance-loading and attenuation effects. Moreover, because the attenuation effects of electronic processing vary with frequency, the usefulness of a coaxial cable delay line over a wide range of frequencies is suspect. Consequently, there is a need for a delay line which performs well over a range of signals including high speed signals.
In a time-multiplexing application, the outputs of n parallel channels are multiplexed to provide a single serial channel, where the bit rate, f, of the individual channels is converted into a stream having a bit rate of nf. This may be accomplished by connecting each of the n signal lines to a delay line and combining the outputs from the delay lines into a single channel. The delay lines are designed so that the delays introduced in successive lines increase linearly. The increment in delay-line length is the product of the velocity of light multiplied by the period of the desired signal stream.
A commonly used technique to create delay lines for time-multiplexing applications is to use a series of fixed length delay lines. For any particular arrangement of fixed-length delay lines, the frequency of the multiplexed signals must be kept constant. However, in many applications (e.g. digital testing), it is desirable to provide the capability of varying the frequency of the multiplexed signal. Consequently, there is a need for a multiplexer which can operate over a variety of frequencies.